Control systems



Nov. 18. 1969 c. F. ALSOP 3,478,551

CONTROL SYSTEMS Filed May 4, 1967 2 Sheets-Sheet 1 N Qmulllllillllllllllilllllllllllfl Q2655 o @zEmw "Em d8 mm mm Q mm A W 5&62mm 4 I I. N l. 3: MW ,mfiwfiz :2: $3 m: mm X mw mm R Qm & Q 8 Fl i I Ii l l I I l l I I I I i i I I I l l i ll C. F. ALSOP CONTROL SYSTEMSNov. 18, 1969 Filed May 4, 1967 2 Sheets-Sheet 2 M Q 7 l I l I l l I I ll l I I Q2353 $255 m new :8 9 u .8 m \\\Nk \I\! P 535% qfiz 1 QZRWDEQQmm XII .w IWQ m5 :3 m M m Xv a t \g x QM a r I l I l I I l I I I I I l lI I I l l I i I I I I United States Patent 3,478,551 CONTROL SYSTEMSCharles Francis Alsop, Baslow, Bakewell, England, assignor to Davy andUnited Instruments Limited, Sheffield, Yorkshire, England Filed May 4,1967, Ser. No. 636,109 Claims priority, application Great Britain, May6, 1966, 20,138/ 66 Int. Cl. B21b 37/12 US. Cl. 728 8 Claims ABSTRACT OFTHE DISCLOSURE This invention relates to control systems for controllingrolling mills. It is particularly concerned with such a control system,in which the roll gap setting is controlled by signals generated independence on the rolling load and the roll gap setting, in order tomaintain the gauge of the material leaving the rolling millsubstantially at a constant value. By roll gap setting is meant the rollgap under zero rolling load conditions, the roll gap setting beingmeasured in a conventional mill stand by an indicator coupled to thescrews or other device supplied for controlling the roll gap.

As described for example in the paper by R. B. Sim-s and P. R. A. Briggsin Sheet Metal Industries March 1954, the rolling load signal (F) andthe roll gap setting (So) are combined according to the equation M beingthe spring coefficient of the mill and h the gauge required of the stripleaving the mill stand. The error signal is applied to control thedevice adjusting the roll gap setting.

Errors in the thickness of the work leaving the rolling mill stand mayarise from two sources. The first is the non-uniformity of the workentering the mill; the thickness, hardness and temperature may vary. Thesecond is the mill stand itself, which, because of the eccentricity ofthe rolls, the non-circularity of the rolls and the nature of the oilfilm bearings for the rolls, may cause cyclic variations in the roll gapdimension at the frequency of revolution of the rolls, or a harmonicthereof.

The control system referred to above is designed to compensate forthickness variations arising from the work itself (referred tohereinafter as work variations) and operates to produce, on thedetection of a roll load change in a given direction, a further changein rolling load in the same direction; for example, if the thickness ofthe strip entering the mill increases, the fact is detected by a rise inthe rolling load and causes a compensating decrease in the roll settingwhich in turn causes a further increase in the rolling load, the sum ofthe aggregate change of rolling load divided by the spring coefiicient(M) and the aggregate change of roll setting being zero for constantoutgoing thickness. The rolling load feed back-loop through the materialbeing rolled the rolling load detector, the computing system and theroll gap setting adjusting device is thus a positive feed backloop.

If now this control system is faced with thickness variations arisingfrom the mill (hereinafter called cyclic variations, the system operatesto accentuate the thickness errors imprinted in the outgoing strip. Forexample, if due to eccentricity, the roll gap decreases with the effectof reducing the material thickness, a rise in rolling load is detectedand the control system responds to the detected change by decreasing theroll gap and thus further reducing the material thickness. If theresponse ice of the control system is sufficiently high therefore, thesystem will not only not compensate for cyclic variations but willaccentuate their effect on the thickness of the strip leaving the mill.The higher the response of the control system is made, the greater arethe thickness variations produced in the strip by cyclic variations, andit is this factor that has imposed a limit on the system response in thepast and hence the efiiciency with which the system can compensate forwork variations.

In a conventional mill stand, the roll gap adjusting rolling mill stand,in which signals are generated in accordance with the rolling load andthe roll gap setting and these signals are used jointly to control meansfor adjusting the roll gap setting, has a path for the rolling loadsignal to the roll gap adjusting means such that a negative feed-backcontrol loop is constituted for rolling load variations at frequenciesof the same order as the cyclic variations, but a positive feed-backcontrol loop is constituted for rolling load variations at lowerfrequencies.

In a conventional zmill stand, the roll gap adjusting means are thescrewdowns which act between the housings and the roll ends. In othermill stands, however, the roll gap adjusting means may take other forms;for example they may be constituted by wedges between the housings androlls; or, in the case of prestressed mills, =by hydraulic cylinderswhich apply an external force to put the stand parts into compression ortension, and which are adjustable to vary the elongation of the standand thus the roll gap; or, by position regulated jacks acting on therolls in place of the conventional screwdowns. The roll gap adjustingmeans usually consist of two devices, one acting on each side of themill, and actuating mechanisms for those devices and it is desirable tocrosscouple the two devices, or their actuating mechanisms, to ensurethat the two devices move together uniformly; thus a signal,representing the position and/ or velocity of the device, may be takenfrom each device or actuating mechanism and fed into the complementarydevice or mechanism so that both devices are maintained in closesynchronism both dynamically and statically.

In a preferred form of the invention, the "automatic control systemcomprises means for generating a signal in accordance with the rollingload of the stand, means for generating a signal in accordance with theroll setting, a first path for applying the rolling load signal tocontrol the roll gap adjusting means in a direction such that any changein rolling load gives rise to a change in roll setting causing a changeof the rolling load in the same direction, a second path in parallelwith the first path for applying the rolling load signal to theadjusting means in a direction such that a change in rolling load causesa change in rolling load in the opposite direction, means for applyingthe roll setting signal to the adjusting means in conjunction with therolling load signal of the path, and frequency sensitive filter means torender the first path substantially unetfective at frequencies of thesame order as the cyclic variations, the arrangement being such that,for variations at frequencies of the same order as the cyclicvariations, the adjusting means is controlled to maintain the rollingload substantially constant, while, at lower frequencies, the adjustingmeans is controlled jointly by the roll setting signal and rolling loadsignal to compensate substantially for work variations.

By the invention, it is possible simultaneously to have a high systemresponse and to reduce the gauge variations resulting from eccentricity.

The invention will be more readily understood by way of example from thefollowing description of a rolling mill gauge control system inaccordance therewith reference being made to the drawing accompanyingthe pro companying drawing (FIGURE 3), in which:

FIGURE 1 schematically illustrates a rolling mill for strip metal,

FIGURE 2 schematically illustrates the control svstem. and

FIGURE 3 shows a modification of FIGURE 2.

.In FIGURE 1, the rolling mill 12 is schematically illustrated by a pairof rolls 13, and screws 14 driven by a motor 15 to adjust the gapbetween the rolls and hence the reduction performed on the strip 16passing between the rolls. The roll gap setting is measured by apotentiometer 16 or other position transducer, coupled to the screws,while load cells 17 or other force transducers interposed between thescrews 14 and the roll chocks measure the rolling load. The roll gapsetting is the gap between the rolls for zero rolling load and can beconveniently measured in a conventional mill by the potentiometer 16; ina prestressed mill, in which certain mill components are subject to aforce other than the rolling load, the potentiometer signal may requireto be adjusted to take account of the change in the roll gap due to thatother force.

While, for simplicity, screws are shown for adjusting the roll gapsetting, other adjusting mechanisms such as wedges, may be used, aposition transducer being coupled to the mechanism in the manner of thepotentiometer 16. In the conventional mill illustrated, the two screws14 are coupled together so as to be equally operated by motor 15. In aprestressed mill, the roll gap adjustment may be effected by more thantwo screws or like devices, which may or may not be commonly driven orby variation of the prestress pressure. In all cases, however, the rollgap adjusting means at the two sides of the mill stand must be connectedtogether so as to move uniformly, by actuating them from a common drive,by connecting together the drives when independent, by having a commonsupply of liquid under pressure in the case of a prestressed mill or byelectrically cross-connecting the drives, specially when hydraulicdrives are employed. By cross-connecting the drives is meant that asignal, representing position and/ or velocity of the adjusting device,is taken from each device and is fed into the control for thecomplementary device, in order that both devices may be maintained inclose synchronism both dynamically and statically.

Turning to FIGURE 2, the signals from the load cells 17 are summed togive an average value for the rolling load F and are applied in parallelto two paths. The first path contains a circuit 20 which modifies theload signal by a factor K. The modified signal is applied through anadding circuit 21 where the roll gap setting signal is added in, afurther adding circuit 22, a limiting circuit 23 and an integratingcircuit 24, to a further adding circuit 25, the output from which isapplied through a switch 26 to control the screw motor 15. A feed-backpath is supplied by the line 27 from the output of integrator 24 to theadding circuit 22.

The second path consists only of a circuit 28 which modifies the loadsignal by an adjustable factor K The output of circuit 28 is applied toadding circuit 25.

Adding circuit has three further inputs. A first is supplied from anX-ray gauge 30 which as shown in FIG- URE 1 is located downstream of themill and which generates a signal representing the error between theactual gauge rolled and the required gauge. A second input is providedby a potentiometer 31 which is set by the operator to the initial rollgap setting. The third input is derived from a potentiometer 32 drivenby a servo-motor 33 which can be connected through switch 26 to theoutput of the adding circuit 25.

Disregarding for the moment the limiting circuit 23, the integrator 24and the second path constituted by circuit 28, the control circuitillustrated in FIGURE 2 is the well-known Gaugemeter circuit. The factorK is chosen to be equal to the compliance of the mill (l/M), so that thesum of the signals from circuit 20 and the potentiometer 16 representstheoutgoing gauge of the strip. This is compared in circuit 25 with thesignal set on potentiometer 31 by the operator and the resulting errorsignal is applied to control the screws 14 to reduce that error signaltowards zero. If the resulting strip gauge differs from that required,the X-ray gauge 30 detects an error and applies an added referencesignal to circuit 25, tending to bring the gauge nearer the requiredvalue. If now the operator changes over the switch 26, the residualerror is applied to servo-motor 33 which drives potentiometer 32 toreduce that residual error to zero. Switch 26 is then changed back tothe position shown, when the system continues to control to the gaugeset on the X-ray gauge.

For the reasons explained above, the control system so far described,however high its response, i unable to compensate for roll and rollbearing eccentricity and noncircularity, and it is for this purpose thatthe elements 23, 24 and 28 are introduced. Whereas the rolling load paththrough the circuit 20' has a positive feed-back characteristic, thepath through circuit 28 has a negative characteristic, K Taking the twopaths together, the positive feed-back gain is (K K and the values areselected so that (K -K is equal to the mill compliance (l/M).

The circuit 23 has a limiting characteristic having a positive output ofconstant value for all inputs exceeding a set value and a negativeoutput of constant value for all negative inputs exceeding a set value.When a large input signal is applied to limiting circuit 23, theintegrator gives a ramp output having a maximum rate limited by the setvalue. Under this circumstance, the integrators output increases atuniform rate so long as a positive signal is applied to circuit 23 andthen decreases at uniform rate so long as a negative signal is applied.The feed-back path 27 ensures that under steady state conditions theoutput of integrator 24 will equal the input to circuit 23. The eifectof the circuits 23, 24 is to cause the output of integrator 24 to followthe input to circuit 23 with accuracy when the rate of change of theinput is equal to or less than the maximum ramp rate of circuits 23, 24,but to attenuate input signals, the rate of change of which is greaterthan the maximum ramp rate. The constants of the circuits are selectedso that signals at the frequencies of the cyclic variations aresubstantially attenuated, while those at the smaller frequencies ofpossible variations in the incoming strip sufier hardly any attenuation.

It will be appreciated that the circuits 22, 24, 27 thus act as a lowpass filter, and in fact any circuit which has the describeddifferential attenuating function may be used in their place.

When the load cells 17 detects a variation in rolling load at a low rateof change, due to changes in the characteristics of the ingoing strip,both paths are eifective and the system operates with a positivefeed-back gain of (K +K )=1/M in the manner described. On the otherhand, cyclic variations give rise to load cell signal variations whichpass unattenuated only through the path containing the circuit 28; therolling load loop then acts as a negative feed-back loop of gain K andby appropriate selection of the value of K may operate to compensate andlargely remove variations in output gauge due to this cause. For thispurpose, the roll gap adjusting mechanism must be fast acting inrelation to the strip speed and, for high strip speeds, should be devoidof inertial lags; for this reason hydraulic actuating adjustingmechanisms are preferred.

As will be appreciated the filter constituted by the elements 22, 23, 24is effective, at the frequency of the cyclic variations, on both therolling load signal from load cells 17 and the roll setting signal fromindicator 26. In the modification of FIGURE 3, that filter is replacedby two low pass filters 34, 35, each of which may be constituted by theelements 22, 23, 24, 27 as described, and

one 34 of which is connected between the modifying circuit 20 and theadding circuit-21 so as to be effective only on the rolling load signal,and-the other 35 of which is connected between the indicator 16 and theadding circuit 21, to be effective only on the roll setting signal. Thefunction of the two separate filters 34, 35 is identical with thatdescribed for the single filter 22, 23, 24, 27 of FIGURE 2, but theselectivities of the two filtersmay be chosen to have different values.Then, if the rolling load feed-back loop through load cells 17 and theroll setting feed-back loop through indicator 16 are found to havedifferent stabilities, the selectivity of one filter can be made higherthan that of the other to give maximum overall sensitivity withoutdanger of instability.

An alternative system to attain similar results may be to permit thefirst AGC positive feedback loop through circuit 20 to be operative atall useful frequencies and to introduce a frequency selective circuitinto the negative loop, through circuit 28, this frequency selectiveloop .to comprise a high pass filter. In this case, the gain of circuit20, K is equal to 1AM, while the loop gain at eccentricity frequenciesis (Kg-K1).

Advantageously, the feed-back loop gain (K -K in the first case and K inthe second case may be adjusted, preferably automatically to compensatefor roll flattening, certain speed effects and for changes in the widthof the strip-rolled. In addition, it is preferable to adjustautomatically the pass band of the frequency selective circuit inaccordance with a function of the speed of the strip 16, so thatcompensation is made for the variation with speed in the frequencies ofthe cyclic variations.

I claim:

1. In a rolling mill comprising a pair of work rolls definingtherebetween a roll gap, and signal-responsive means for adjusting thesetting of said roll gap;

a rolling mill control system comprising means for generating a signalwhich varies with the rolling load applied to said rolls by the workpassing therebetween, and

transmission means for transmitting said signal to said adjusting meansin order to control said adjusting means, I

said transmission means comprising means for modifying said signal, whenany variations in said signal are generated at frequencies of the sameorder as the cyclic variations aIiSing from the mill, to produce achange in the transmitted signal in a direction opposite to that of thechange, but for producing a change in said transmitted signal in thesame direction as the change when such variations are generated at lowerfrequencies.

2. In a rolling mill stand comprising a pair of work rolls definingtherebetween a roll gap for the work to be rolled, and means foradjusting the setting of said roll p;

an automatic control system comprising means for generating a rollingload signal in accordance with the rolling load generated in said standby the work,

means for generating a signal in accordance with said roll gap setting,

and signal transmitting means comprising a first conductive path forapplying said rolling load sig-. nal to the roll gap adjusting means inafdirection such that any change in rolling load gives rise to a changein roll setting causing a change of the rolling load in the samedirection as the change, 1

a second conductive path in parallel with the first path for applyingsaid rolling load signal to said adjusting means in a direction suchthat a change in rolling load causes a change in rolling load in theopposite direction,

means for applying said roll gap setting signal to said adjusting meansin conjunction with the rolling load signal transmitted by said firstpath, and frequency sensitive filter means to render said first pathsubstantially ineffective at frequencies of the same order as cyclicvariations in said rolling load signal arising from the stand, where-'by for variations at frequencies of the same order as the cyclicvariations, the adjusting means is controlled to maintain the rollingload substantially constant, while, at lower frequencies, the adjustingmeansis controlled jointly by the roll setting signal and rolling loadSignal to compensate substantially for work variations.

3. An automatic control system for a rolling mill stand according toclaim 2 in which only said first path includes said filter means, andthe roll setting signal applying means. are eifective to apply thatsignal also to the said filter means. 7

4. Anautomatic control system for a rolling mill stand according toclaim 2 in which the filter means comprise a first filter element in thefirst path and effective on the rolling load signal only and a secondfilter element in the roll setting signal applying means.

5. An automatic control system for a rolling mill stand according toclaim 4 in which the two filter elements have dilfering selectivities.

6. An automatic control system for a rolling mill stand according toclaim 3 in which each of the paths contains a circuit element alteringthe characteristic of the rolling load signal whereby, at frequenciesbelow the frequencies of the cyclic variations, the effect of the twopaths is to modify the rollingload signal by a factor approximating tothe compliance of the mill.

7. An automatic control system for a rolling mill stand according toclaim 2, in which the roll gap adjusting means comprise separate deviceseffective on adjustment of the roll setting at the respective roll ends,and those devices are cross-connected to ensure equality of movement.

8. In a rolling mill stand comprising a pair of work rolls definingtherebetween a roll gap for the work to be rolled, and means foradjusting the setting of said roll p;

an automatic control system comprising means for generating a rollingload signal in accordance with the rolling load generated in said standby the work,

means for generating a signal in accordance with said roll gap setting,and

signal transmitting means comprising a first conductive path forapplying said rolling load signal to control the roll gap adjustingmeans in a direction such that any change in rolling load gives rise toa change in roll setting causing a change of the rolling load in thesame direction as the change,

a second conductive path in parallel with the first path. for applyingsaid rolling load signal to said adjusting means in a direction suchthat a change in rolling load causes a change in rolling load in theopposite direction,

means for applying said roll setting signal to said adjusting means inconjunction with the rolling load signal passed by said first path,

and frequency sensitive filter means to render said second pathsubstantially ineffective at frequencies of a lower order than those ofcyclic variations arising from the stand, whereby for variations atfrequencies of the same order as the cyclic variations, the adjustingmeans is controlled to maintain the rolling load substantially constant,while,

7 8 at lower frequencies, the adjusting means 3,177,346 4/1965 Green 728is' controlled jointly by the roll setting sig- 3,186,200 6/ 1965MaxWel1 728 rial and rolling load signel to compensate 3,287,94611/1966" Perraultet a1. 728 I substannally for work vanetlons. OR IPATENTS References Cited 125,389 1 /1966 Canada.

UNITED STATES PATENTS MILTON Primary Examiner 8/1966 Wright 728 A a

